1. Geological Museum, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
2. Danish Lithosphere Center, Øster Voldgade 10, 1350 Copenhagen K, Denmark

Correspondence to: Martin Bizzarro^1,2 Correspondence and requests for materials should be addressed to M.B. (e-mail: Email: mbi@dlc.ku.dk).

Abstract

The ¹⁷⁶Lu to ¹⁷⁶Hf decay series has been widely used to understand the nature of Earth's early crust–mantle system^{1, 2, 3, 4, 5, 6}. The interpretation, however, of Lu–Hf isotope data requires accurate knowledge of the radioactive decay constant of ¹⁷⁶Lu (_¹⁷⁶Lu), as well as bulk-Earth reference parameters. A recent calibration of the _¹⁷⁶Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9 Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3 Gyr ago⁷, but does not provide evidence for a complementary depleted mantle reservoir. Here we report Lu–Hf isotope measurements of different Solar System objects including chondrites and basaltic eucrites. The chondrites define a Lu–Hf isochron with an initial ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.279628 0.000047, corresponding to _¹⁷⁶Lu = 1.983 0.033 10^-11 yr^-1 using an age of 4.56 Gyr for the chondrite-forming event. This _¹⁷⁶Lu value indicates that Earth's oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment⁷. The depletion event must have occurred no later than 320 Myr after planetary accretion, consistent with timing inferred from extinct radionuclides^{8, 9, 10}.